In order to achieve certain physical-mechanical properties in metal coatings, which can be deposited electrolytically, certain additive compounds must be added in small amounts to the deposition solution. Of main concern in this respect are organic materials, which have an effect on the bright finish, the levelling and the uniformity of the deposition on large surfaces, avoidance of so-called burnt-on particles, i.e. deposition of granular crystalline coatings and also the construction of metal coatings with high fracture elongation and tensile strength.
The disadvantage in this respect is that these materials generally disintegrate during deposition, so that they have to be replenished during the operation. Admittedly, the observation of constant conditions in production is mostly very difficult, since the materials themselves are only present in very small concentrations in the deposition solutions, and in addition a complicated mixture of several materials of this type is also most often required to achieve certain coating properties and finally during dissolution degraded products are formed also, which have an effect on the metal coating properties. Therefore, an analytical survey of the additive compounds is not only very difficult, but is generally not adequate either for completely describing the state of the deposition bath, with the result that analytical methods for controlling the bath may only be used in a qualified manner.
In addition there is a demand, in the coating of complex shaped workpieces for example of circuit boards with very fine borings, for achieving as uniform a thickness of metal coating as possible on all points of the workpiece. It is possible, with appropriate deposition baths with optimised composition, to enlarge the metal coating thickness even in places with a low current density. However, the named additive compounds only influence the metal dispersion so slightly that the problem was not solved by these optimising measures.
In particular, it was not possible with the named measures to achieve even an adequately uniform distribution of metal coating thickness in complex shaped workpieces, for example in circuit boards with very fine borings.
Various means of solving the problem have been suggested therefore in the literature, however none of them have yet led to entirely satisfactory solutions.
As a solution for the equalising of the metal distribution on the surfaces of the workpieces which are to be coated, the use of insoluble anodes during metal deposition is suggested. Anodes of this type are known from the German Patent document DD 215 589 B5 and in the publication DD 261 613 A1. Furthermore methods of this type are also described in DE 43 44 387 A1. In these publications mention is made also of the addition of compounds of electrochemically reversible redox systems to the depositing solution, with which systems the addition of metal salts for completing the deposited metal ions should be avoided.
A periodic current reversal during electrolysis is suggested as a further solution for equalising the coating thickness on the workpieces ("Pulse Plating-Elektrolytische Metallabscheidung mit Pulsstrom", Ed. Jean-Claude Puippe and Frank Leaman, Eugen G. Leuze Verlag, Saulgau, Germany, 1986, p.26 and "Pulse Plating of Copper for Printed Circuit Technology" M. R. Kalantary, D. R. Gabe, Metal finishing 1991 pp. 21 to 27). However, adequate uniformity of the deposited metal coatings cannot be achieved in this way on large and in addition complex shaped workpieces.
Furthermore, in DE 27 39 427 C2 a method for uniform coating of profiled workpieces, which have narrow recesses, is described. For this purpose, the recesses in the surface of the workpiece are treated very intensively with electrolytic solution and, at the same time, an electric cycle of pulses lasting from 1 .mu.sec to 50 .mu.sec with considerably larger breaks in between was applied to the workpiece. This procedure is very costly however, since a targeted injection at the profiles in workpiece surfaces is not possible at least for mass production or it requires a very high industrial fitting cost.
In WO-89/07162 A1 an electrochemical method for depositing metals is described, preferably for copper from a sulphuric acid copper electrolyte with organic additive compounds for improving the physical-mechanical properties of workpieces, for example circuit boards. For this purpose, alternating current with varying long cathodic and anodic pulses is used. Coatings are successfully deposited on complex shaped workpieces, such as for example circuit boards, with more uniform coating thickness. Further indications for overcoming the problem of avoiding varying coating thicknesses by altering the geometric ratios in the electrolytic cell, for example by dissolving the anodes, are not offered.
When the experiments described there were repeated, an improvement in the metal dispersion in the fine borings of circuit boards could only be achieved, in our own findings, if at the same time, the optical appearance of the copper coating, deposited according to the method described in the publication, became worse. Furthermore, the ductility of these coatings was so slight that, even when a coated circuit board with borings was immersed just once for ten seconds into a 288.degree. C. hot soldering bath, fractures in the copper coating, particularly at the transition from the circuit board surface to the boring wall, could be seen.
In the essay "Hartverchromung mittels eines Gleichrichters mit pulsierenden Wellen und periodischer Umkehr der Polaritat" by C. Colombini in the Professional Paper Galvanotechnik, 1988, pp. 2869 to 2871 a method is likewise described, in which the metal coatings are deposited not by direct current but rather by pulsating alternating current. According to the author's proposal this serves for producing chrome coatings, which are more corrosion proof than traditional coatings. Admittedly, it states in this publication that chrome coatings, formed according to this method, are grey and do not shine, with the result that they must be polished subsequently to produce a glossy surface. Apart from the fact that a subsequent mechanical treatment of this type is very exacting and therefore very expensive, in many cases this cannot be carried out at all, for example when the surface spots, which are to be treated, are not accessible.
In the essay "Pulse Reverse Copper Plating for Printed Circuit Boards" by W. F. Hall et al., Proc. of the American Electrochemical Society, 10.sup.th Plating in the Electronic Industry Symposium, San Francisco, Calif., February 1983 it has been shown, furthermore, that copper coatings, which have been deposited by means of a pulsing current method, can be formed on circuit boards with a more uniform coating thickness from depositing solutions with brighteners, as with copper coatings which are deposited by direct current.
The copper coatings are matt, according to information in the publication, partly even brown or orange and do not consist thus of pure copper. To this extent it is surprising that, according to the author's data, high ductility values, namely high fracture elongation values, and tensile strength values, can be achieved using the method indicated. However, no adequately precise data about the deposition conditions, as, for example, the bath composition, temperature of the bath or the anodes used are given.
In EP 0 356 516 A1 a device for depositing electroplated coatings is shown, with which the physical-mechanical properties of the coatings can be improved, in the view of the inventor. For this purpose, the amplitude, shape and frequency of the currents flowing through the electrolytic bath during deposition are automatically changed. It is also stated that, by measuring and stabilising the current in the electroplating bath during deposition of the electroplated coatings, the physical-mechanical properties are likewise improved.
In EP 0 129 338 B1 a method for electrolytic treatment of the surface of a metal rail using graphite electrodes is described, in which, by using alternating current with asymmetrical positive and negative half-waves during the electrolytic treatment, the dissolution of the graphite electrodes used as anodes can be avoided, with the result that the distribution of current in the graphite electrode no longer alters and constant conditions can be maintained during electrolysis. Admittedly, no indications are given in this publication of how an improvement can be achieved in the physical-mechanical properties of deposited metal coatings and, at the same time, how to make the distribution of coating thickness as uniform as possible over a long period of operation.